The purpose of this three year phase III study is to investigate the therapeutic effect of Ibandronate in the long term treatment of postmenopausal osteoporosis by evaluating the efficacy of long term intermittent and long term continuous oral treatment using the incidence of new vertebral fractures as the primary endpoint. The specific objectives are to determine whether Ibandronate prevents fractures and bone loss in women with established postmenopausal osteoporosis, and to determine the type of side effects associated with the oral administration of Ibandronate.